They'd have to build a dome because Vegas is in the middle of the freaking desert. Every time I've gone I've managed to get dehydrated and I drink a lot of water as it is. I would also imagine players wouldnt be so happy with the idea of playing in those conditions. They build an open-air stadium in that city and the team continues to stink they wouldn't be able to give away tickets. It would be like TTUN moved out west.
Another problem with this idea, Lake Mead is drying up. That lake dries up Vegas will be a ghost town. Ironically, the Raiders would be the perfect team for a ghost town lol.